Metabolic modifiers are compounds that are either fed to animals or implanted to improve
rate of gain, improve feed efficiency, dressing per cent, carcass meat yield, visual meat
quality, shelf-life or meat palatability. Most metabolic modifiers have been developed
and researched to improve growth performance and carcass composition, with fewer
of them developed or researched specifically to improve meat quality. Several in-depth
review papers have been written on specific categories or types of metabolic modifiers.
This chapter presents a general review of the effects of metabolic modifiers on carcass
composition, fresh meat quality and cooked meat palatability. Emphasis will be on those
metabolic modifiers that are approved for use in cattle in the United States and other
developed countries. The following are the main types of modifier discussed in this
chapter:

2.1 Introduction to anabolic steroid implants
Anabolic implants in beef production systems have been studied extensively since their
approval for use by the Federal Drug Administration in the 1950s (FDA, 2014). Several
products are currently available for use in the United States and several developed
countries, each consisting of various compounds and concentrations for use at different
stages of animal growth. Most steroids are administered as subcutaneous implants placed
in the middle third of the ear, usually as compressed pellets, but some as silastic rods.
Release time for most implants range from 50 to 200 days. Approximately 97% of cattle in
the United States entering the feedlot have received at least one anabolic implant during
their production lives (NAHMS, 2000). Anabolic steroid implants are characterized as
oestrogenic, androgenic or a combination of both oestrogenic and androgenic compounds
(combination implants). Additionally, these compounds may also be categorized as natural
hormones and xenobiotics, or combinations of these. Products available for use in beef
cattle production, including doses and sex-specific usage, have been summarized by
others (Montgomery et al., 2001; Duckett and Pratt, 2014); see Table 1, which includes
the abbreviations used in this chapter. If trade names are not declared, steroid and
concentrations used will be reported.